Various radiation curable, and particularly ultraviolet (UV) light curable, ink compositions are known in the art. By nature, UV curable inks are primarily thermoset systems. The thermoset nature of these materials makes it difficult to formulate inks that exhibit high elongation upon curing and formability, i.e. by thermoforming, vacuum forming, or the like. This difficulty can be alleviated to a degree by using high molecular weight oligomers which can improve the elongation and formability of UV cured materials. However, the addition of high molecular weight oligomers increases the viscosity of the liquid materials which prohibits their use in jettable applications in where viscosities are typically below 70 cPs at 25° C. While solvent-based, air cured jettable inks have been proposed, where the viscosity of the polymer or oligomer is reduced with a solvent, such inks require unpractical drying times and can be unfriendly to the environment due to high volatile organic component content.
UV curable screen inks have been used, however, screen ink processes require building a new screen each time the printed image changes. An operator does not have the ability to change the image on demand, as is the case with a digital inkjet printing process.
Other considerations are also important in formulating inks to satisfy numerous criteria affecting ink performance and stability. For example, the inks must possess an appropriate level of surface tension, low volatility, low smear, high image quality (especially at high print speeds), and adhesion to a variety of substrate materials. Stability of the inks is also important, including storage stability, stability at high shear rates, stability at high temperatures, and stability at the extreme conditions inside a print head, e.g. a piezoelectric or thermal head. Also desired is the elimination of volatile solvents from the inks. Current commercially available UV curable inkjet inks are limited in one or more of these areas.
US Pub. No. 2006/0222831 discloses curable ink compositions comprising an oligomeric component, which has a molecular weight of more than about 10,000 g/mole and which does not contain radiation curable ethylenically unsaturated functionality, a diluent, additives, and a mono-functional monomer having a molecular weight of 100 g/mol to 600 g/mol. US Pub. No. 2006/0275588 is a Continuation-in-Part of US Pub. No. 2006/0222831, and discloses curable ink compositions additionally comprising an acrylic polymer or copolymer having an acid number of below about 20. The compositions of the present invention differs from these disclosures in that they do not include the described high molecular weight non-reactive oligomer, do not require the presence of a solvent and, by contrast, the oligomer component included in the compositions of the invention, contains radiation curable, ethylenically unsaturated functionality.
Accordingly, a need exists for further improvements in radiation curable and jettable inks.